This invention relates to the reduction of dislocations in semiconductor layers epitaxially grown on mismatched substrates and, more particularly, to double heterostructure junction lasers incorporating such layers.
In the summer of 1970 I. Hayashi and M. B. Panish reported successful c.w. operation at room temperature of a semiconductor p-n junction laser now known as the double heterostructure (DH) laser (Applied Physics Letters, Vol. 17, No. 3, pages 109- 111, Aug. 1, 1970). This achievement was the result of intensive efforts both in physics, which led to the design of the DH laser, and in chemistry, which led to improved liquid phase epitaxy (LPE) techniques for realizing the DH laser.
Briefly, the DH laser comprises a narrow bandgap active region which may be either n-type, p-type or may contain a p-n junction in which case it has both conductivity types. The active region is sandwiched between relatively wider bandgap, opposite-conductivity-type layers which form two heterojunctions, one at each interface with the active region. These heterojunctions, as is now well known, serve to confine injected carriers as well as stimulated radiation to the active region. Consequently, it was early recognized by Hayashi and Panish that the heterojunctions should have as few defects as possible because such defects could act as nonradiative recombination centers which would tend to reduce efficiency and increase lasing thresholds. They, therefore, fabricated their DH lasers by LPE from the GaAs-AlAs semiconductor system; i.e., early forms of the DH lasers comprised an n-GaAs substrate on which were grown the following layers: n-Al.sub.x Ga.sub.1.sub.-x As, p-GaAs (the active region) and p-Al.sub.x Ga.sub.1.sub.-x As. Because GaAs and AlAs are nearly lattice matched at the growth temperature (about 800.degree. C), GaAs and AlGaAs were even better lattice matched, so that particularly good heterojunctions were formed during LPE fabrication.
When operated c.w. at room temperature, however, these early forms of the DH laser typically had relatively short lifetimes ranging from only a few minutes to tens of hours. Consequently, systematic studies of DH lasers were undertaken in order to identify degradation mechanisms and to develop solutions to the short lifetime problem.
For example, R. L. Hartman and A. R. Hartman have shown that the reduction of process-induced stress (e.g., stress due to bonding of contacts) increases the lifetime of DH lasers (see Applied Physics Letters, Vol. 23, p. 147 (1973)). Panish and Rozgonyi have suggested that the reduction of growth-induced stress may further extend the lifetime (see the concurrently filed application, Case 11-6, supra). More particularly, the latter application teaches that although GaAs and AlAs are nearly lattice matched at the LPE growth temperature, their lattice mismatch is magnified at room temperature (where the lasers are intended to operate) because of the different thermal expansion coefficients of the two materials. Therefore, at room temperature there is considerable stress in the grown epitaxial layers.
Panish and Rozgonyi propose that the process be reversed; i.e., that a slight lattice mismatch be intentionally introduced at the growth temperature so that the difference in thermal expansion, before a deleterious factor, can be taken advantage of to produce a better lattice match at room temperature where DH lasers are most often operated. They teach that the average stress at room temperature between contiguous layers of Al.sub.x Ga.sub.1.sub.-x As and Al.sub.y Ga.sub.1.sub.-y As (0 .ltoreq. x,y .ltoreq. 1; y &gt; x) can be reduced by the addition of small, controlled amounts of phosphorus during the growth of the latter layer so that the quaternary Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z is grown instead of the ternary Al.sub.y Ga.sub.1.sub.-y As. They have found that the amount of phosphorus added must be properly chosen: too little added may reduce stress by only a minimal amount whereas too much added may in fact increase stress and may generate dislocations. In particular, they teach that in order to reduce the average stress at room temperature to less than about 2 .times. 10.sup.8 dynes/cm.sup.2 the amount of phosphorus added should satisfy the condition: EQU 0.03 .ltoreq. z/(y-x) .ltoreq. 0.05, (1)
and to reduce the average stress to substantially zero the following condition should be satisfied: EQU z/(y-x) = .perspectiveto. 0.04. (2)
However, growth with reduced strain does not necessarily eliminate substrate dislocations which extend into the quaternary layer. Such dislocations can expand to the point where the operation of a junction laser will terminate (see Applied Physics Letters, Vol. 23, p. 469 (Oct. 15, 1973) by P. M. Petroff and R. L. Hartman).